As a post VS2010 SP1 installation activity, I am recompiling all my open source projects.

The first is XHTMLFIX at http://xhtmlfix.codeplex.com/

This LGPL project has simple fixes to ASP.NET 2.0/4.0 to achieve XHTML compliance as measured by the W3C tests at http://validator.w3.org/

The XHTML project shows as untrue the commonly held belief that MVP or MVC are necessary for producing XHTML compliant web pages. Incidentally the other supposed advantage of MVP and MVC over web forms of easier testing is also very dubious as web forms can be tested by systems such as Selenium or WaTiN. I have used NUnitASP (alas sadly discontinued) with web forms and found it be more effective than unit testing MVP. Now if you prefer the MVP and / or MVC approach over Web forms then fine, that is your preferance.

Now if you can find an example where ASP.NET 4.0 Web forms properly written do not produce XHTML compliant markup, I would be glad of your example and will look at ways of modifying the markup to be XHTML compliant.